Many flexible print circuit boards (hereinafter, FPC) and flexible flat cables (hereinafter, FFC) for use in an electric appliance or the like use copper or copper alloy for their wiring. When they are electrically connected with other circuit boards or the like, there have been employed connector connection in many cases. In such a connection, in order to reduce a connection resistance of the connector with the wiring thus to avoid conduction faults, a surface-treatment is carried out for a copper wiring terminal. An example thereof includes an electrolytic plating treatment with gold, tin-lead alloy, or the like. However, gold plating is accompanied by a problem of high costs. In addition, plating with lead-containing alloys has a problem of environmental destruction through elution of lead therefrom, which motivates a practical use of lead-free plating. To this end, there has been investigated a plating treatment with pure tin or lead-free tin alloys. However, when a copper wiring terminal plated with lead-free tin alloys or pure tin is fitted with a connector, it is confirmed that a whisker-like crystal, simply called whisker, rapidly develops from a contact portion therebetween, since a connecter pin is pressed on the wiring terminal portion. It has also been revealed that such phenomenon is significant in pure tin plating or lead-free tin alloy plating. According to further experiments, it has been confirmed that the phenomenon occurs when pressure greater than a certain magnitude is applied even after a plated film is plastically deformed. Growth of such whiskers is undesirable since there is caused a short-circuit between copper wirings or somewhere, leading to a trouble in electric appliances or the like.
A technique for solving a whisker problem is disclosed in a Japanese Patent Application Laid-open Publication No. H11-343594. Namely, this publication discloses a configuration in which a CuSn containing layer or a CuSn containing alloy layer is formed above a base substance having at least a surface of Cu or Cu alloy with an intermediate layer therebetween, the intermediate layer being composed of Cu6Sn5 (η′ phase) and Cu3Sn (ε phase) laminated in this order. These layers each contain 0.1 to 3 wt % of Cu and have a thickness of 1 to 20 μm. These layers are made by plating the base substance having at least a surface of Cu or Cu alloy in an Sn plating bath or an Sn alloy plating bath containing Cu ions of 0.2 to 50 ppm. In addition, the publication describes that whisker development can be almost prevented by reflow treatment. However, it has been revealed that the reflow treatment alone has a prevention effect for naturally developing whiskers but not a sufficient effect for whiskers developed as result of fitting the connector.
Japanese Patent Publication No. 3014814 discloses a method of preventing tin plating whisker that may cause a short-circuit in a minute pattern of copper or copper alloy wiring electro-plated with tin. Specifically, in the method, first of all, a pure tin layer having a thickness of 0.1 μm or more is formed on a minute pattern of copper or its alloy by plating; next, the pure tin layer is entirely turned into a Cu—Sn diffusion layer by thermal treatment; and last a pure tin plating layer having a thickness of 0.15 to 0.8 μm is formed thereon. This prevents development of tin plating whiskers. However, since the tin plating is conducted twice in this method, there is caused a problem of complicated procedures, production costs or the like. Therefore, there has been desired a technique for solving these problems.